wikiheaders: bridge wiki Category docs to the headers!

Did an initial cleanup on the headers and wrote a few pieces of documentation,
but this needs more work to fill out the documentation.

include/SDL3/SDL_stdinc.h

@@ -20,11 +20,11 @@
 */
 
 /**
- *  \file SDL_stdinc.h
+ * # CategoryStdinc
  *
- *  This is a general header that includes C language support. It implements
- *  a subset of the C runtime: these should all behave the same way as their
- *  C runtime equivalents, but with an SDL_ prefix.
+ * This is a general header that includes C language support. It implements a
+ * subset of the C runtime: these should all behave the same way as their C
+ * runtime equivalents, but with an SDL_ prefix.
  */
 
 #ifndef SDL_stdinc_h_